1. Field of The Invention
This invention relates to a method for applying protective silicone coatings to substrates. More particularly, this invention involves heating a specified class of moisture reactive organosiloxane compositions to a flowable state and applying the compositions as coatings to a substrate, whereupon the compositions cool and become non-flowable. The initially applied coatings can be made flowable by heating but in the presence of atmospheric moisture gradually convert to a resin-reinforced elastomeric material that is not reflowable with heating to temperatures well above the application temperature. Many of the coatings can be handled at ambient temperature within minutes after being coated without damage to the coating or transfer of coating material.
The cured coatings provide long term protection for substrates against harsh environments at temperatures above the application temperature of the initial coating.
2. Background Information
Many combinations of resinous and liquid polymers are known. Some of these exhibit the properties characteristic of pressure sensitive adhesives, also referred to as PSA's.
PSA's based on organosilicon compounds typically contain 1) a silicone resin containing specified concentrations of monofunctional R.sub.3 SiO units, referred to as M units, and tetrafunctional SiO.sub.4/2 units, referred to as Q units, and 2) a high molecular weight polydiorganosiloxane having the consistency of a gum. These compositions are typically applied as a solution in a compatible organic liquid or a low viscosity, liquid nonreactive polyorganosiloxane. Following application of the solution to a desired substrate the liquid is evaporated to deposit the PSA.
U.S. Pat. No. 2,814,601, issued to Currie et al. describes curable silicone PSA's useful for bonding silicone rubber to metals such as aluminum, and which cure to a permanent adhesive at room temperature even in the absence of moisture.
U.S. Pat. No. 4,865,920, which issued to R. Sweet, describes solventless silicone PSA's that can be applied as a heated melt, also referred to as a hot melt. The compositions differ from prior art PSA's by replacement of the gum type polydiorganosiloxane with a liquid polydiorganosiloxane and by addition of from 1 to 10 percent, based on the combined weight of the organosilicon compounds, of a liquid ester of a monocarboxylic acid containing from 2 to 32 carbon atoms. Replacing this ester with a phenyl-containing polysiloxane fluid to make the PSA more compatible with drugs and other organic materials is described in U.S. Pat. No. 5,162,410, which issued to R. Sweet.
The hot melt PSA's described in both Sweet patents do not contain moisture activated reactive groups capable of forming a cured material, and therefore retain the flowability at elevated temperatures and the tackiness characteristic of PSA's.
A commonly assigned copending application, Ser. No. 07/748,848, filed on Aug. 22, 1991 describes silicone PSA's that cure in the presence of moisture to permanent adhesives. The compositions are particularly useful as structural adhesives for adhering glass panels to structural materials such as concrete, aluminum and steel. The two required ingredients of the PSA are an MQ resin and a high consistency polydiorganosiloxane gum wherein at least 50 percent and preferably at least 90 percent of the silicon-bonded hydrocarbon radicals are methyl.
One or both ingredients of the PSA compositions described in the aforementioned copending application contain moisture activated curing groups of the formula R.sub.b (OR').sub.3-b SiZ--, where R represents a monovalent hydrocarbon radical, R' represents an alkyl or aryl radical, Z is a divalent group that durably links the curing group to a silicon atom of one of the PSA ingredients and the value of h is 0 or 1. Typical linking groups that can be represented by Z include but are not limited to oxygen, one or more diorganosiloxane units, and hydrocarbylene radicals that optionally contains one or more hetero atoms selected from oxygen, nitrogen and sulfur.
Due to the high viscosity of gum type polydiorganosiloxanes even at elevated temperatures, the moisture curable PSA compositions are not suitable for application as heated molten materials, but instead are applied to substrates as solutions in a miscible organic solvent such as xylene. The solvent is removed prior to curing of the composition or evaporates during the curing reaction.
Japanese laid open patent application (Kokai) No. 4(1992)-/81,487 describes curable pressure sensitive adhesive compositions comprising (1) 100 parts by weight of an organosiloxane copolymer consisting essentially of triorganosiloxy and SiO.sub.4/2 units and up to 0.7 weight percent of silanol groups, where the molar ratio of triorganosiloxy to SiO.sub.4/2 units is from 0.6 to 1.2, respectively, (2) a liquid polydiorganosiloxane containing as the terminal units alkoxy, alkenyloxy, iminoxy, acyloxy and/or aminoxy radicals, and (3) a condensation catalyst to promote curing of the composition in the presence of atmospheric moisture. The molar ratio of silanol groups in the resinous copolymer to terminal units in the liquid polydiorganosiloxane is from 1 to 10.
The characterizing feature of the moisture reacting compositions described in the Japanese patent application is their ability to retain the characteristics of pressure sensitive adhesives under both dry and wet conditions following reaction of the moisture reactive groups. This requires that the molar ratio of silanol units in the resinous copolymer to terminal units in the liquid polydiorganosiloxane be from 1 to 10, and preferably from 4 to 8.
U.S. Pat. No. 5,091,484, which issued to A. Colas on February 25, 1992 describes elastomer-forming compositions containing (1) a hydroxyl- or alkoxy-terminated polydiorganosiloxane (2) an alkoxy-functional MQ type silicone resin that is preferably liquid under ambient conditions and (3) a titanium-containing curing catalyst. These compositions are described as flowable under ambient conditions and cure in the presence of atmospheric moisture to yield elastomeric materials.
Moisture curable elastomer-forming compositions containing (1) a hydroxyl-terminated polydiorganosiloxane (2) an MQ type silicone resin containing from 0.5 to 6 weight percent of hydroxyl groups, (3) alkoxy-functional organosilicon compounds as the curing agent, and (4) a titanium-containing curing catalyst are described in U.S. Pat. No. 4,143,088, which issued to G. Favre et al. on Mar. 6, 1979.
Organic polymer compositions that are solid under ambient conditions and sufficiently flowable when heated to be applied as coatings that can subsequently be cured by a free radical reaction are described in U.S. Pat. No. 4,990,364, which issued to Bolte et al., U.S. Pat. No. 5,070,121, which issued to Hinterwaldner et al., and Japanese Laid Open Application 89/251,837.
Moisture curable organic polymer compositions that form conventional or pressure sensitive adhesives and can be applied as heated melts are described in Published European Patent Application No. 0420864. The compositions comprise a silane-modified copolymer of ethylene and a polar monomer such as vinyl acetate, a catalyst and an optional plasticizer. The modified polymers contain silicon-bonded hydrolyzable groups.
Patent application WO 91/08420, filed under the provisions of the Patent Cooperation Treaty, describes adhesive compositions containing a tackifier resin or tackifier wax that has been modified by reaction with a silane containing hydrolyzable groups that are present in the final resin or wax. The composition optionally contains an organic polymer with groups capable of reacting with the silicon-bonded hydrolyzable groups to form a crosslinked material.
When curable compositions applied in the molten state are used as protective coatings, the cooled uncured compositions should preferably exhibit sufficient adhesion and sufficiently low tackiness to allow the coated articles to be handled immediately or shortly following application of the coating composition without damage to the coating or transfer of coating material to objects that contact the articles.
When the substrate is a printed circuit board, to correct coating defects and make repairs to the electronic components mounted on the coated substrate it is desirable that at least the interior portion of the coating remain reflowable upon heating for a limited time period while curing under ambient conditions to ultimately form a crosslinked, non-reflowable elastomeric material exhibiting excellent adhesion to the substrate, as demonstrated by a resistance to corrosion of metal surfaces and articles on the substrate.
With the increasing emphasis on eliminating or at least reducing the amount of volatile materials released during the application and curing of coating compositions, there has been a growing demand for curable compositions that can be applied in the absence of solvents or diluents.
It is therefore an objective of the present invention to provide moisture reactive coating compositions that are substantially free of unreactive liquid materials and can be heated for application as molten liquids that upon contacting a substrate at ambient temperature rapidly form solid coatings exhibiting strong adhesion to clean or contaminated substrates without requiring priming or other surface preparation. The initially applied coatings are reflowable upon heating and in the presence of moisture under ambient conditions are gradually converted to substantially tack-free resin-reinforced elastomeric materials that are not reflowable even at temperature substantially above their application temperature and maintain their initial adhesion to the substrate.